The present invention relates to the field of data transmission, particularly with respect to fiber optic transmission systems, such as Dense Wavelength Division Multiplexing (DWDM) systems.
Wavelength Division Multiplexing (WDM) is a fiber-optic transmission technique that uses optical wavelengths to transmit multiple channels of data and/or analog signals. Within optical networks, WDM allows the transmission over the optical layer of e-mail, video and multimedia, carried as Internet Protocol (IP) data over Asynchronous Transfer Mode (ATM), and voice, carried over Synchronous Optical Networks/Synchronous Digital Hierarchy (SONET/SDH). Despite the fact that these formatsxe2x80x94IP, ATM and SONET/SDHxe2x80x94provide unique bandwidth management capabilities, all three can be transported over the optical layer using WDM. A particular example of WDM is Dense Wavelength Division Multiplexing (DWDM), offering greater capacity in terms of the number of channels transmitted as well as a greater speed of transmission.
DWDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific frequencies (wavelengths or lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity. Each signal carried can be at a different rate (OC-3, OC-12, OC-48, OC-192, etc.) and in a different format (SONET, ATM, data, etc.). For example, a DWDM network with a mix of SONET signals operating at OC-48 (2.5 Gbps) and OC-192 (10 Gbps) over a DWDM infrastructure can achieve capacities of over 40 Gbps.
A DWDM system can be viewed as a parallel set of optical channels, each using a slightly different light wavelength, but all sharing a single transmission medium. Several elements may have an impact on the performance of such a system, notably optical power, linear noise, non-linear distortion, mismatches between the transmitter and receiver, among others. With the evolution of the DWDM system towards more channels, narrower channel spacing and higher speed, performance monitoring and optimization of the system becomes more and more challenging.
Unfortunately, traditional system optimization methods, such as Optical Signal-to-Noise Ratio (OSNR), demonstrate various weaknesses such that they are unable to provide the necessary overall estimation of the system performance. More specifically, while OSNR has a short run time and can be applied when the system is in-service, it only considers linear noise in the system and ignores the non-linear distortion difference among the wavelength division multiplexed channels.
Another existing optimization method is the Sequential Tx Attenuation and Rx Test (START), which uses the Bit Error Ratio (BER) margin to optimize the DWDM system performance. START considers both linear and non-linear noise but is very time consuming and can only be used in the initial system tune up, as opposed to having a more practical in-service application.
The background information provided above clearly indicates that there exists a need in the industry to provide an improved method and apparatus for optimizing the performance of an optical transmission system, in particular a DWDM system.
According to a first aspect, the present invention provides a tuning system for use in a data receiver of an optical transmission system. The optical transmission system can be a DWDM system, among other possibilities. The tuning system includes a first input for observing a signal transported on a data channel of the data receiver and a second input for observing a signal transported on a monitoring channel of the data receiver. The data channel and the monitoring channel carry the same information. The data channel can be viewed as a primary data transport channel, while the monitoring channel carries a copy of the data and makes that copy available to the tuning system for analysis.
The tuning system includes a processing module that is coupled to the first and to the second inputs. Based on information received at the first and second inputs, the processing module generates a control signal conveying tuning information. The tuning information can be used by the data receiver to alter its operating point such as to enhance or improve its performance on the data channel of the data receiver.
An advantage of this approach is to allow, when required or desired, tuning of the system while the system is in service. This is possible since the tuning of the system is performed using the monitoring channel, and does not disturb nor corrupt the actual data being transported on the data channel.
In a specific example of implementation, the tuning information conveyed in the control signal allows the data receiver to alter a decision threshold used to discriminate one binary value from another binary value on the data channel. For example, the decision threshold can be a voltage value. When the signal on the data channel is above this voltage value, the data receiver will interpret the signal as one binary value (1 or 0). Conversely, when the signal on the data channel is below this voltage value, the data receiver will interpret the signal as another binary value (0 or 1).
Continuing with this example of implementation, the tuning system includes an additional output to generate another control signal for selectively altering the decision threshold applied on the copy of the data signal transported on the monitoring channel. The processing module sequentially sets the decision threshold to different positions (in terms of voltage values, this implies that the reference voltage is set to different values) such as to artificially change the bit error rate on the monitoring channel. Note that this operation is conducted only at the monitoring channel, the data on the data channel being left undisturbed.
The alteration of the decision threshold is made such as to induce in the monitoring channel two different bit error rate values, namely 1.0E-7 and 1.0E-8. Each bit error rate value is associated to two different decision thresholds. In other words, a first deviation of the decision threshold in one direction (say an increase) will result in a certain bit error rate value for the channel, and a second deviation of the decision threshold in the other direction (a decrease) will result in the same bit error rate value for the channel, where the first and second deviations may be of different sizes. Thus two different decision thresholds can yield the same absolute bit error rate.
The bit error rate can be computed by comparing the data that is artificially corrupted on the monitoring channel, by performing decision threshold variations, to the actual data transported on the data channel.
The processing module is operative to compute a Q factor for each bit error rate value. This allows the processing unit to compute four pairs of values, each pair including a Q factor and a decision threshold. Recall that under this example, each bit error rate value, and hence each Q factor, is associated to two different decision thresholds, which yields four pairs of values. By applying a linear function to the four pairs, the decision threshold that corresponds to the optimum Q factor can be extrapolated. This extrapolated decision threshold is truly for the monitoring channel since it has been established on the basis of data observed on the monitoring channel. However, the behavior of the data channel being identical or almost identical to the monitoring channel, the same decision threshold generally applies as well.
The decision threshold is then applied to the data channel by issuing a control signal associated with the data channel that conveys the new decision threshold to be applied to the data signal.
In a different aspect, the invention provides an adjustment system designed to perform Q factor tuning in a multi-channel receive unit. Each channel of the receive unit is provided with a data receiver. The Q factor of a channel can be selectively altered independent of the Q factors of the other channels. The tuning system measures, derives or assesses the Q factor for each channel and issues a control signal to selectively alter Q factors as desired.
In a specific example of implementation, the Q factor for each data receiver is obtained from the tuning system of the data receiver, that computes the Q factor, as described in general terms earlier. The adjustment system attempts to perform Q factor equalization such that the Q factor of each channel falls within a certain Q factor range. Specifically, if the Q factor for a particular channel is outside the Q factor range, the adjustment system will issue a control signal which causes alteration of the Q factor of the particular channel. For instance, the control signal may be designed to regulate the power of the data signal transported in the channel such as to alter the Q factor as desired. The power regulation can be accomplished by conveying the control signal to the transmit end of the channel where the data signal originates and where a power control mechanism can be easily implemented.
The invention also provides a method for tuning a data receive unit in an optical transmission system.